Aromatic carboxylic acids are useful chemical compounds and are raw materials for a wide variety of manufactured articles. For example, terephthalic acid (TA) is manufactured on a world-wide basis in amounts exceeding 10 billion pounds per year. A single manufacturing plant can produce 100,000 to more than 750,000 metric tons of TA per year. TA is used, for example, to prepare polyethylene terephthalate (PET), a raw material for manufacturing polyester fibers for textile applications and polyester film for packaging and container applications. TA can be produced by the high pressure, exothermic oxidation of a suitable aromatic feedstock compound, such as para xylene, in a liquid-phase reaction using air or other source of molecular oxygen as the oxidant and catalyzed by one or more heavy metal compounds and one or more promoter compounds. Methods for oxidizing para xylene and other aromatic compounds using such liquid-phase oxidations are well known in the art. For example, Saffer in U.S. Pat. No. 2,833,816 discloses a method for oxidizing aromatic feedstock compounds to their corresponding aromatic carboxylic acids. Central to these processes for preparing aromatic carboxylic acids is the use of a liquid-phase reaction employing a low molecular weight carboxylic acid, such as acetic acid, as part of the reaction solvent. A certain amount of water is also present in the oxidation reaction solvent and water is also formed as a result of the oxidation reaction. The oxidation reaction also produces a reaction offgas which generally comprises carbon dioxide, carbon monoxide and, depending on the promoter used, may contain methyl bromide. Additionally, when air is used as the source of molecular oxygen, the reaction offgas contains nitrogen gas and unreacted oxygen. Although various means can be used to control the temperature of the highly exothermic oxidation reaction, it is generally most convenient to remove heat by allowing the solvent to vaporize, i.e. boil, during the oxidation reaction. The vaporized solvent, which is typically a mixture of water and low-molecular weight carboxylic acid, has heretofore been condensed in one or more overhead condenser apparatus and the condensate returned to the reaction mixture. However, since water is also present, at least part of the condensate is usually directed to a separation apparatus, typically a distillation column, to separate the water from the low molecular weight aliphatic acid solvent so that the water concentration in the reactor is maintained at a constant level. The offgases that are not condensed are typically vented or passed through an oxidizer to burn byproducts and form an environmentally acceptable effluent.
The high pressure offgas contains a considerable amount of energy. Although prior art processes have, to an extent, utilized some of the energy contained in the offgas by running the offgas through, for example, an expander or turbine, prior art processes did not fully utilize the energy available in this high pressure offgas. In prior processes, heat removal from the reaction mixture was accomplished by condensing a portion of the reaction overhead vapor to produce moderate pressure steam. The moderate pressure steam, in part, was used to recover energy by a steam turbine, and a part was used to separate water from acetic acid by distillation.
The art, therefore, needs an improved process to manufacture aromatic carboxylic acids wherein the energy generated by the highly exothermic, high pressure oxidation reaction can be economically and efficiently recovered and, in fact, serve as a net generator of energy. The present invention provides such an improved process.